1. Field of the Invention
The present invention relates to a liquid ejection head for ejecting liquid such as ink.
2. Description of the Related Art
In recent years, there has been an increasing demand for image forming apparatus including inkjet printers that can operate at higher speed and produce better quality images than ever. To realize image forming apparatus that operate at higher speed and produce higher quality images, more ejection ports for ejecting liquid such as ink need to be formed at the nozzle plate of the liquid ejection head that the image forming apparatus includes. For this reason, a plurality of ejection ports are highly densely arranged at the nozzle plate of the liquid ejection head of any image forming apparatus that has been developed in recent years. As a result of densely arranging ejection ports, the liquid ejection head has been made to form better quality images than ever.
When ejection ports are densely arranged on the nozzle plate of a liquid ejection head, for example, at a rate of 600 dpi or 1,200 dpi, the gaps separating the ejection ports are inevitably reduced. Then, by turn the wall separating the ejection ports are inevitably made thin to consequently reduce the strength of the entire nozzle plate. As a result, the entire nozzle plate may probably be deformed by the stress given rise to by thermal energy to be used for ejecting liquid such as ink from the ejection ports. Such deformation will be particularly remarkable in a middle part of the nozzle plate as viewed in the running direction of the rows of ejection ports. Additionally, as larger liquid ejection heads come into use, the stress due to thermal energy tends to be concentrated in a middle part of the nozzle plate as viewed in the direction of the rows of ejection ports. As the nozzle plate is deformed, the ejection ports formed in the nozzle plate, or at least some of them, become deformed so that liquid may be ejected from the nozzle plate in directions other than the proper direction.
Japanese Patent Application Laid-Open No. 2000-158657 discloses a liquid ejection head including a nozzle plate (ejection port plate) having projections formed at positions located vis-à-vis the supply ports (ink supply ports) of the substrate body of the liquid ejection head for supplying liquid to the ejection ports (ink ejection ports) so as to project toward the substrate body. As the nozzle plate is provided with such projections, the rigidity of the entire nozzle plate is raised to make the nozzle plate less liable to be deformed.
Japanese Patent Application Laid-Open No. 2007-283501 discloses a liquid ejection head including a nozzle plate (ejection port plate) having pillar-shaped projections projecting from the projections (beam-like projections), which beam-like projections project into the common liquid chamber to supply the ejection ports with liquid, toward the ejection ports. As the nozzle plate is provided with such pillar-shaped projections projecting from the beam-like projections in addition to the beam-like projections arranged at the nozzle plate, the rigidity of the entire nozzle plate is further raised to make the nozzle plate much less liable to be deformed.
However, while the rigidity of the entire nozzle plate is raised by either of the inventions disclosed in Japanese Patent Application Laid-Open No. 2000-158657 and Japanese Patent Application Laid-Open No. 2007-283501, the problem that the stress due to thermal energy tends to be concentrated in a middle part of the nozzle plate as viewed in the direction of the rows of ejection ports is left unresolved. Therefore, if the liquid ejection head is operated for a long period of time and hence thermal energy is generated repeatedly, the nozzle plate of the liquid ejection head becomes liable to be deformed in a middle part of the nozzle plate as viewed in the direction of the rows of ejection ports by the stress due to thermal energy. As a middle part of the nozzle plate as viewed in the direction of the rows of ejection ports is deformed, the ejection ports at the middle part of the nozzle plate are also deformed so that liquid may no longer be ejected from the nozzle plate in the proper direction. Then, consequently the quality of the image formed by the liquid ejection head will inevitably be degraded.
On the other hand, if the rigidity of the entire nozzle plate is raised further by arranging larger projections on the nozzle plate in order to prevent a middle part of the nozzle plate as viewed in the direction of the rows of ejection ports from being deformed, the stress is concentrated not only to the nozzle plate itself but also to the surface of the nozzle plate and that of the substrate body that are bonded to each other. Then, the nozzle plate can come off from the substrate body to allow liquid to leak out from the gap between the substrate body and the nozzle plate. Then again, consequently the quality of the image formed by the liquid ejection head will inevitably be degraded.